Source file src/cmd/compile/internal/syntax/parser.go

Documentation: cmd/compile/internal/syntax

     1  // Copyright 2016 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package syntax
     6  
     7  import (
     8  	"fmt"
     9  	"go/build/constraint"
    10  	"io"
    11  	"strconv"
    12  	"strings"
    13  )
    14  
    15  const debug = false
    16  const trace = false
    17  
    18  type parser struct {
    19  	file  *PosBase
    20  	errh  ErrorHandler
    21  	mode  Mode
    22  	pragh PragmaHandler
    23  	scanner
    24  
    25  	base      *PosBase // current position base
    26  	first     error    // first error encountered
    27  	errcnt    int      // number of errors encountered
    28  	pragma    Pragma   // pragmas
    29  	goVersion string   // Go version from //go:build line
    30  
    31  	top    bool   // in top of file (before package clause)
    32  	fnest  int    // function nesting level (for error handling)
    33  	xnest  int    // expression nesting level (for complit ambiguity resolution)
    34  	indent []byte // tracing support
    35  }
    36  
    37  func (p *parser) init(file *PosBase, r io.Reader, errh ErrorHandler, pragh PragmaHandler, mode Mode) {
    38  	p.top = true
    39  	p.file = file
    40  	p.errh = errh
    41  	p.mode = mode
    42  	p.pragh = pragh
    43  	p.scanner.init(
    44  		r,
    45  		// Error and directive handler for scanner.
    46  		// Because the (line, col) positions passed to the
    47  		// handler is always at or after the current reading
    48  		// position, it is safe to use the most recent position
    49  		// base to compute the corresponding Pos value.
    50  		func(line, col uint, msg string) {
    51  			if msg[0] != '/' {
    52  				p.errorAt(p.posAt(line, col), msg)
    53  				return
    54  			}
    55  
    56  			// otherwise it must be a comment containing a line or go: directive.
    57  			// //line directives must be at the start of the line (column colbase).
    58  			// /*line*/ directives can be anywhere in the line.
    59  			text := commentText(msg)
    60  			if (col == colbase || msg[1] == '*') && strings.HasPrefix(text, "line ") {
    61  				var pos Pos // position immediately following the comment
    62  				if msg[1] == '/' {
    63  					// line comment (newline is part of the comment)
    64  					pos = MakePos(p.file, line+1, colbase)
    65  				} else {
    66  					// regular comment
    67  					// (if the comment spans multiple lines it's not
    68  					// a valid line directive and will be discarded
    69  					// by updateBase)
    70  					pos = MakePos(p.file, line, col+uint(len(msg)))
    71  				}
    72  				p.updateBase(pos, line, col+2+5, text[5:]) // +2 to skip over // or /*
    73  				return
    74  			}
    75  
    76  			// go: directive (but be conservative and test)
    77  			if strings.HasPrefix(text, "go:") {
    78  				if p.top && strings.HasPrefix(msg, "//go:build") {
    79  					if x, err := constraint.Parse(msg); err == nil {
    80  						p.goVersion = constraint.GoVersion(x)
    81  					}
    82  				}
    83  				if pragh != nil {
    84  					p.pragma = pragh(p.posAt(line, col+2), p.scanner.blank, text, p.pragma) // +2 to skip over // or /*
    85  				}
    86  			}
    87  		},
    88  		directives,
    89  	)
    90  
    91  	p.base = file
    92  	p.first = nil
    93  	p.errcnt = 0
    94  	p.pragma = nil
    95  
    96  	p.fnest = 0
    97  	p.xnest = 0
    98  	p.indent = nil
    99  }
   100  
   101  // takePragma returns the current parsed pragmas
   102  // and clears them from the parser state.
   103  func (p *parser) takePragma() Pragma {
   104  	prag := p.pragma
   105  	p.pragma = nil
   106  	return prag
   107  }
   108  
   109  // clearPragma is called at the end of a statement or
   110  // other Go form that does NOT accept a pragma.
   111  // It sends the pragma back to the pragma handler
   112  // to be reported as unused.
   113  func (p *parser) clearPragma() {
   114  	if p.pragma != nil {
   115  		p.pragh(p.pos(), p.scanner.blank, "", p.pragma)
   116  		p.pragma = nil
   117  	}
   118  }
   119  
   120  // updateBase sets the current position base to a new line base at pos.
   121  // The base's filename, line, and column values are extracted from text
   122  // which is positioned at (tline, tcol) (only needed for error messages).
   123  func (p *parser) updateBase(pos Pos, tline, tcol uint, text string) {
   124  	i, n, ok := trailingDigits(text)
   125  	if i == 0 {
   126  		return // ignore (not a line directive)
   127  	}
   128  	// i > 0
   129  
   130  	if !ok {
   131  		// text has a suffix :xxx but xxx is not a number
   132  		p.errorAt(p.posAt(tline, tcol+i), "invalid line number: "+text[i:])
   133  		return
   134  	}
   135  
   136  	var line, col uint
   137  	i2, n2, ok2 := trailingDigits(text[:i-1])
   138  	if ok2 {
   139  		//line filename:line:col
   140  		i, i2 = i2, i
   141  		line, col = n2, n
   142  		if col == 0 || col > PosMax {
   143  			p.errorAt(p.posAt(tline, tcol+i2), "invalid column number: "+text[i2:])
   144  			return
   145  		}
   146  		text = text[:i2-1] // lop off ":col"
   147  	} else {
   148  		//line filename:line
   149  		line = n
   150  	}
   151  
   152  	if line == 0 || line > PosMax {
   153  		p.errorAt(p.posAt(tline, tcol+i), "invalid line number: "+text[i:])
   154  		return
   155  	}
   156  
   157  	// If we have a column (//line filename:line:col form),
   158  	// an empty filename means to use the previous filename.
   159  	filename := text[:i-1] // lop off ":line"
   160  	trimmed := false
   161  	if filename == "" && ok2 {
   162  		filename = p.base.Filename()
   163  		trimmed = p.base.Trimmed()
   164  	}
   165  
   166  	p.base = NewLineBase(pos, filename, trimmed, line, col)
   167  }
   168  
   169  func commentText(s string) string {
   170  	if s[:2] == "/*" {
   171  		return s[2 : len(s)-2] // lop off /* and */
   172  	}
   173  
   174  	// line comment (does not include newline)
   175  	// (on Windows, the line comment may end in \r\n)
   176  	i := len(s)
   177  	if s[i-1] == '\r' {
   178  		i--
   179  	}
   180  	return s[2:i] // lop off //, and \r at end, if any
   181  }
   182  
   183  func trailingDigits(text string) (uint, uint, bool) {
   184  	i := strings.LastIndexByte(text, ':') // look from right (Windows filenames may contain ':')
   185  	if i < 0 {
   186  		return 0, 0, false // no ':'
   187  	}
   188  	// i >= 0
   189  	n, err := strconv.ParseUint(text[i+1:], 10, 0)
   190  	return uint(i + 1), uint(n), err == nil
   191  }
   192  
   193  func (p *parser) got(tok token) bool {
   194  	if p.tok == tok {
   195  		p.next()
   196  		return true
   197  	}
   198  	return false
   199  }
   200  
   201  func (p *parser) want(tok token) {
   202  	if !p.got(tok) {
   203  		p.syntaxError("expected " + tokstring(tok))
   204  		p.advance()
   205  	}
   206  }
   207  
   208  // gotAssign is like got(_Assign) but it also accepts ":="
   209  // (and reports an error) for better parser error recovery.
   210  func (p *parser) gotAssign() bool {
   211  	switch p.tok {
   212  	case _Define:
   213  		p.syntaxError("expected =")
   214  		fallthrough
   215  	case _Assign:
   216  		p.next()
   217  		return true
   218  	}
   219  	return false
   220  }
   221  
   222  // ----------------------------------------------------------------------------
   223  // Error handling
   224  
   225  // posAt returns the Pos value for (line, col) and the current position base.
   226  func (p *parser) posAt(line, col uint) Pos {
   227  	return MakePos(p.base, line, col)
   228  }
   229  
   230  // errorAt reports an error at the given position.
   231  func (p *parser) errorAt(pos Pos, msg string) {
   232  	err := Error{pos, msg}
   233  	if p.first == nil {
   234  		p.first = err
   235  	}
   236  	p.errcnt++
   237  	if p.errh == nil {
   238  		panic(p.first)
   239  	}
   240  	p.errh(err)
   241  }
   242  
   243  // syntaxErrorAt reports a syntax error at the given position.
   244  func (p *parser) syntaxErrorAt(pos Pos, msg string) {
   245  	if trace {
   246  		p.print("syntax error: " + msg)
   247  	}
   248  
   249  	if p.tok == _EOF && p.first != nil {
   250  		return // avoid meaningless follow-up errors
   251  	}
   252  
   253  	// add punctuation etc. as needed to msg
   254  	switch {
   255  	case msg == "":
   256  		// nothing to do
   257  	case strings.HasPrefix(msg, "in "), strings.HasPrefix(msg, "at "), strings.HasPrefix(msg, "after "):
   258  		msg = " " + msg
   259  	case strings.HasPrefix(msg, "expected "):
   260  		msg = ", " + msg
   261  	default:
   262  		// plain error - we don't care about current token
   263  		p.errorAt(pos, "syntax error: "+msg)
   264  		return
   265  	}
   266  
   267  	// determine token string
   268  	var tok string
   269  	switch p.tok {
   270  	case _Name, _Semi:
   271  		tok = p.lit
   272  	case _Literal:
   273  		tok = "literal " + p.lit
   274  	case _Operator:
   275  		tok = p.op.String()
   276  	case _AssignOp:
   277  		tok = p.op.String() + "="
   278  	case _IncOp:
   279  		tok = p.op.String()
   280  		tok += tok
   281  	default:
   282  		tok = tokstring(p.tok)
   283  	}
   284  
   285  	// TODO(gri) This may print "unexpected X, expected Y".
   286  	//           Consider "got X, expected Y" in this case.
   287  	p.errorAt(pos, "syntax error: unexpected "+tok+msg)
   288  }
   289  
   290  // tokstring returns the English word for selected punctuation tokens
   291  // for more readable error messages. Use tokstring (not tok.String())
   292  // for user-facing (error) messages; use tok.String() for debugging
   293  // output.
   294  func tokstring(tok token) string {
   295  	switch tok {
   296  	case _Comma:
   297  		return "comma"
   298  	case _Semi:
   299  		return "semicolon or newline"
   300  	}
   301  	return tok.String()
   302  }
   303  
   304  // Convenience methods using the current token position.
   305  func (p *parser) pos() Pos               { return p.posAt(p.line, p.col) }
   306  func (p *parser) error(msg string)       { p.errorAt(p.pos(), msg) }
   307  func (p *parser) syntaxError(msg string) { p.syntaxErrorAt(p.pos(), msg) }
   308  
   309  // The stopset contains keywords that start a statement.
   310  // They are good synchronization points in case of syntax
   311  // errors and (usually) shouldn't be skipped over.
   312  const stopset uint64 = 1<<_Break |
   313  	1<<_Const |
   314  	1<<_Continue |
   315  	1<<_Defer |
   316  	1<<_Fallthrough |
   317  	1<<_For |
   318  	1<<_Go |
   319  	1<<_Goto |
   320  	1<<_If |
   321  	1<<_Return |
   322  	1<<_Select |
   323  	1<<_Switch |
   324  	1<<_Type |
   325  	1<<_Var
   326  
   327  // advance consumes tokens until it finds a token of the stopset or followlist.
   328  // The stopset is only considered if we are inside a function (p.fnest > 0).
   329  // The followlist is the list of valid tokens that can follow a production;
   330  // if it is empty, exactly one (non-EOF) token is consumed to ensure progress.
   331  func (p *parser) advance(followlist ...token) {
   332  	if trace {
   333  		p.print(fmt.Sprintf("advance %s", followlist))
   334  	}
   335  
   336  	// compute follow set
   337  	// (not speed critical, advance is only called in error situations)
   338  	var followset uint64 = 1 << _EOF // don't skip over EOF
   339  	if len(followlist) > 0 {
   340  		if p.fnest > 0 {
   341  			followset |= stopset
   342  		}
   343  		for _, tok := range followlist {
   344  			followset |= 1 << tok
   345  		}
   346  	}
   347  
   348  	for !contains(followset, p.tok) {
   349  		if trace {
   350  			p.print("skip " + p.tok.String())
   351  		}
   352  		p.next()
   353  		if len(followlist) == 0 {
   354  			break
   355  		}
   356  	}
   357  
   358  	if trace {
   359  		p.print("next " + p.tok.String())
   360  	}
   361  }
   362  
   363  // usage: defer p.trace(msg)()
   364  func (p *parser) trace(msg string) func() {
   365  	p.print(msg + " (")
   366  	const tab = ". "
   367  	p.indent = append(p.indent, tab...)
   368  	return func() {
   369  		p.indent = p.indent[:len(p.indent)-len(tab)]
   370  		if x := recover(); x != nil {
   371  			panic(x) // skip print_trace
   372  		}
   373  		p.print(")")
   374  	}
   375  }
   376  
   377  func (p *parser) print(msg string) {
   378  	fmt.Printf("%5d: %s%s\n", p.line, p.indent, msg)
   379  }
   380  
   381  // ----------------------------------------------------------------------------
   382  // Package files
   383  //
   384  // Parse methods are annotated with matching Go productions as appropriate.
   385  // The annotations are intended as guidelines only since a single Go grammar
   386  // rule may be covered by multiple parse methods and vice versa.
   387  //
   388  // Excluding methods returning slices, parse methods named xOrNil may return
   389  // nil; all others are expected to return a valid non-nil node.
   390  
   391  // SourceFile = PackageClause ";" { ImportDecl ";" } { TopLevelDecl ";" } .
   392  func (p *parser) fileOrNil() *File {
   393  	if trace {
   394  		defer p.trace("file")()
   395  	}
   396  
   397  	f := new(File)
   398  	f.pos = p.pos()
   399  
   400  	// PackageClause
   401  	f.GoVersion = p.goVersion
   402  	p.top = false
   403  	if !p.got(_Package) {
   404  		p.syntaxError("package statement must be first")
   405  		return nil
   406  	}
   407  	f.Pragma = p.takePragma()
   408  	f.PkgName = p.name()
   409  	p.want(_Semi)
   410  
   411  	// don't bother continuing if package clause has errors
   412  	if p.first != nil {
   413  		return nil
   414  	}
   415  
   416  	// Accept import declarations anywhere for error tolerance, but complain.
   417  	// { ( ImportDecl | TopLevelDecl ) ";" }
   418  	prev := _Import
   419  	for p.tok != _EOF {
   420  		if p.tok == _Import && prev != _Import {
   421  			p.syntaxError("imports must appear before other declarations")
   422  		}
   423  		prev = p.tok
   424  
   425  		switch p.tok {
   426  		case _Import:
   427  			p.next()
   428  			f.DeclList = p.appendGroup(f.DeclList, p.importDecl)
   429  
   430  		case _Const:
   431  			p.next()
   432  			f.DeclList = p.appendGroup(f.DeclList, p.constDecl)
   433  
   434  		case _Type:
   435  			p.next()
   436  			f.DeclList = p.appendGroup(f.DeclList, p.typeDecl)
   437  
   438  		case _Var:
   439  			p.next()
   440  			f.DeclList = p.appendGroup(f.DeclList, p.varDecl)
   441  
   442  		case _Func:
   443  			p.next()
   444  			if d := p.funcDeclOrNil(); d != nil {
   445  				f.DeclList = append(f.DeclList, d)
   446  			}
   447  
   448  		default:
   449  			if p.tok == _Lbrace && len(f.DeclList) > 0 && isEmptyFuncDecl(f.DeclList[len(f.DeclList)-1]) {
   450  				// opening { of function declaration on next line
   451  				p.syntaxError("unexpected semicolon or newline before {")
   452  			} else {
   453  				p.syntaxError("non-declaration statement outside function body")
   454  			}
   455  			p.advance(_Import, _Const, _Type, _Var, _Func)
   456  			continue
   457  		}
   458  
   459  		// Reset p.pragma BEFORE advancing to the next token (consuming ';')
   460  		// since comments before may set pragmas for the next function decl.
   461  		p.clearPragma()
   462  
   463  		if p.tok != _EOF && !p.got(_Semi) {
   464  			p.syntaxError("after top level declaration")
   465  			p.advance(_Import, _Const, _Type, _Var, _Func)
   466  		}
   467  	}
   468  	// p.tok == _EOF
   469  
   470  	p.clearPragma()
   471  	f.EOF = p.pos()
   472  
   473  	return f
   474  }
   475  
   476  func isEmptyFuncDecl(dcl Decl) bool {
   477  	f, ok := dcl.(*FuncDecl)
   478  	return ok && f.Body == nil
   479  }
   480  
   481  // ----------------------------------------------------------------------------
   482  // Declarations
   483  
   484  // list parses a possibly empty, sep-separated list of elements, optionally
   485  // followed by sep, and closed by close (or EOF). sep must be one of _Comma
   486  // or _Semi, and close must be one of _Rparen, _Rbrace, or _Rbrack.
   487  //
   488  // For each list element, f is called. Specifically, unless we're at close
   489  // (or EOF), f is called at least once. After f returns true, no more list
   490  // elements are accepted. list returns the position of the closing token.
   491  //
   492  // list = [ f { sep f } [sep] ] close .
   493  func (p *parser) list(context string, sep, close token, f func() bool) Pos {
   494  	if debug && (sep != _Comma && sep != _Semi || close != _Rparen && close != _Rbrace && close != _Rbrack) {
   495  		panic("invalid sep or close argument for list")
   496  	}
   497  
   498  	done := false
   499  	for p.tok != _EOF && p.tok != close && !done {
   500  		done = f()
   501  		// sep is optional before close
   502  		if !p.got(sep) && p.tok != close {
   503  			p.syntaxError(fmt.Sprintf("in %s; possibly missing %s or %s", context, tokstring(sep), tokstring(close)))
   504  			p.advance(_Rparen, _Rbrack, _Rbrace)
   505  			if p.tok != close {
   506  				// position could be better but we had an error so we don't care
   507  				return p.pos()
   508  			}
   509  		}
   510  	}
   511  
   512  	pos := p.pos()
   513  	p.want(close)
   514  	return pos
   515  }
   516  
   517  // appendGroup(f) = f | "(" { f ";" } ")" . // ";" is optional before ")"
   518  func (p *parser) appendGroup(list []Decl, f func(*Group) Decl) []Decl {
   519  	if p.tok == _Lparen {
   520  		g := new(Group)
   521  		p.clearPragma()
   522  		p.next() // must consume "(" after calling clearPragma!
   523  		p.list("grouped declaration", _Semi, _Rparen, func() bool {
   524  			if x := f(g); x != nil {
   525  				list = append(list, x)
   526  			}
   527  			return false
   528  		})
   529  	} else {
   530  		if x := f(nil); x != nil {
   531  			list = append(list, x)
   532  		}
   533  	}
   534  	return list
   535  }
   536  
   537  // ImportSpec = [ "." | PackageName ] ImportPath .
   538  // ImportPath = string_lit .
   539  func (p *parser) importDecl(group *Group) Decl {
   540  	if trace {
   541  		defer p.trace("importDecl")()
   542  	}
   543  
   544  	d := new(ImportDecl)
   545  	d.pos = p.pos()
   546  	d.Group = group
   547  	d.Pragma = p.takePragma()
   548  
   549  	switch p.tok {
   550  	case _Name:
   551  		d.LocalPkgName = p.name()
   552  	case _Dot:
   553  		d.LocalPkgName = NewName(p.pos(), ".")
   554  		p.next()
   555  	}
   556  	d.Path = p.oliteral()
   557  	if d.Path == nil {
   558  		p.syntaxError("missing import path")
   559  		p.advance(_Semi, _Rparen)
   560  		return d
   561  	}
   562  	if !d.Path.Bad && d.Path.Kind != StringLit {
   563  		p.syntaxErrorAt(d.Path.Pos(), "import path must be a string")
   564  		d.Path.Bad = true
   565  	}
   566  	// d.Path.Bad || d.Path.Kind == StringLit
   567  
   568  	return d
   569  }
   570  
   571  // ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] .
   572  func (p *parser) constDecl(group *Group) Decl {
   573  	if trace {
   574  		defer p.trace("constDecl")()
   575  	}
   576  
   577  	d := new(ConstDecl)
   578  	d.pos = p.pos()
   579  	d.Group = group
   580  	d.Pragma = p.takePragma()
   581  
   582  	d.NameList = p.nameList(p.name())
   583  	if p.tok != _EOF && p.tok != _Semi && p.tok != _Rparen {
   584  		d.Type = p.typeOrNil()
   585  		if p.gotAssign() {
   586  			d.Values = p.exprList()
   587  		}
   588  	}
   589  
   590  	return d
   591  }
   592  
   593  // TypeSpec = identifier [ TypeParams ] [ "=" ] Type .
   594  func (p *parser) typeDecl(group *Group) Decl {
   595  	if trace {
   596  		defer p.trace("typeDecl")()
   597  	}
   598  
   599  	d := new(TypeDecl)
   600  	d.pos = p.pos()
   601  	d.Group = group
   602  	d.Pragma = p.takePragma()
   603  
   604  	d.Name = p.name()
   605  	if p.tok == _Lbrack {
   606  		// d.Name "[" ...
   607  		// array/slice type or type parameter list
   608  		pos := p.pos()
   609  		p.next()
   610  		switch p.tok {
   611  		case _Name:
   612  			// We may have an array type or a type parameter list.
   613  			// In either case we expect an expression x (which may
   614  			// just be a name, or a more complex expression) which
   615  			// we can analyze further.
   616  			//
   617  			// A type parameter list may have a type bound starting
   618  			// with a "[" as in: P []E. In that case, simply parsing
   619  			// an expression would lead to an error: P[] is invalid.
   620  			// But since index or slice expressions are never constant
   621  			// and thus invalid array length expressions, if the name
   622  			// is followed by "[" it must be the start of an array or
   623  			// slice constraint. Only if we don't see a "[" do we
   624  			// need to parse a full expression. Notably, name <- x
   625  			// is not a concern because name <- x is a statement and
   626  			// not an expression.
   627  			var x Expr = p.name()
   628  			if p.tok != _Lbrack {
   629  				// To parse the expression starting with name, expand
   630  				// the call sequence we would get by passing in name
   631  				// to parser.expr, and pass in name to parser.pexpr.
   632  				p.xnest++
   633  				x = p.binaryExpr(p.pexpr(x, false), 0)
   634  				p.xnest--
   635  			}
   636  			// Analyze expression x. If we can split x into a type parameter
   637  			// name, possibly followed by a type parameter type, we consider
   638  			// this the start of a type parameter list, with some caveats:
   639  			// a single name followed by "]" tilts the decision towards an
   640  			// array declaration; a type parameter type that could also be
   641  			// an ordinary expression but which is followed by a comma tilts
   642  			// the decision towards a type parameter list.
   643  			if pname, ptype := extractName(x, p.tok == _Comma); pname != nil && (ptype != nil || p.tok != _Rbrack) {
   644  				// d.Name "[" pname ...
   645  				// d.Name "[" pname ptype ...
   646  				// d.Name "[" pname ptype "," ...
   647  				d.TParamList = p.paramList(pname, ptype, _Rbrack, true) // ptype may be nil
   648  				d.Alias = p.gotAssign()
   649  				d.Type = p.typeOrNil()
   650  			} else {
   651  				// d.Name "[" pname "]" ...
   652  				// d.Name "[" x ...
   653  				d.Type = p.arrayType(pos, x)
   654  			}
   655  		case _Rbrack:
   656  			// d.Name "[" "]" ...
   657  			p.next()
   658  			d.Type = p.sliceType(pos)
   659  		default:
   660  			// d.Name "[" ...
   661  			d.Type = p.arrayType(pos, nil)
   662  		}
   663  	} else {
   664  		d.Alias = p.gotAssign()
   665  		d.Type = p.typeOrNil()
   666  	}
   667  
   668  	if d.Type == nil {
   669  		d.Type = p.badExpr()
   670  		p.syntaxError("in type declaration")
   671  		p.advance(_Semi, _Rparen)
   672  	}
   673  
   674  	return d
   675  }
   676  
   677  // extractName splits the expression x into (name, expr) if syntactically
   678  // x can be written as name expr. The split only happens if expr is a type
   679  // element (per the isTypeElem predicate) or if force is set.
   680  // If x is just a name, the result is (name, nil). If the split succeeds,
   681  // the result is (name, expr). Otherwise the result is (nil, x).
   682  // Examples:
   683  //
   684  //	x           force    name    expr
   685  //	------------------------------------
   686  //	P*[]int     T/F      P       *[]int
   687  //	P*E         T        P       *E
   688  //	P*E         F        nil     P*E
   689  //	P([]int)    T/F      P       []int
   690  //	P(E)        T        P       E
   691  //	P(E)        F        nil     P(E)
   692  //	P*E|F|~G    T/F      P       *E|F|~G
   693  //	P*E|F|G     T        P       *E|F|G
   694  //	P*E|F|G     F        nil     P*E|F|G
   695  func extractName(x Expr, force bool) (*Name, Expr) {
   696  	switch x := x.(type) {
   697  	case *Name:
   698  		return x, nil
   699  	case *Operation:
   700  		if x.Y == nil {
   701  			break // unary expr
   702  		}
   703  		switch x.Op {
   704  		case Mul:
   705  			if name, _ := x.X.(*Name); name != nil && (force || isTypeElem(x.Y)) {
   706  				// x = name *x.Y
   707  				op := *x
   708  				op.X, op.Y = op.Y, nil // change op into unary *op.Y
   709  				return name, &op
   710  			}
   711  		case Or:
   712  			if name, lhs := extractName(x.X, force || isTypeElem(x.Y)); name != nil && lhs != nil {
   713  				// x = name lhs|x.Y
   714  				op := *x
   715  				op.X = lhs
   716  				return name, &op
   717  			}
   718  		}
   719  	case *CallExpr:
   720  		if name, _ := x.Fun.(*Name); name != nil {
   721  			if len(x.ArgList) == 1 && !x.HasDots && (force || isTypeElem(x.ArgList[0])) {
   722  				// x = name "(" x.ArgList[0] ")"
   723  				return name, x.ArgList[0]
   724  			}
   725  		}
   726  	}
   727  	return nil, x
   728  }
   729  
   730  // isTypeElem reports whether x is a (possibly parenthesized) type element expression.
   731  // The result is false if x could be a type element OR an ordinary (value) expression.
   732  func isTypeElem(x Expr) bool {
   733  	switch x := x.(type) {
   734  	case *ArrayType, *StructType, *FuncType, *InterfaceType, *SliceType, *MapType, *ChanType:
   735  		return true
   736  	case *Operation:
   737  		return isTypeElem(x.X) || (x.Y != nil && isTypeElem(x.Y)) || x.Op == Tilde
   738  	case *ParenExpr:
   739  		return isTypeElem(x.X)
   740  	}
   741  	return false
   742  }
   743  
   744  // VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .
   745  func (p *parser) varDecl(group *Group) Decl {
   746  	if trace {
   747  		defer p.trace("varDecl")()
   748  	}
   749  
   750  	d := new(VarDecl)
   751  	d.pos = p.pos()
   752  	d.Group = group
   753  	d.Pragma = p.takePragma()
   754  
   755  	d.NameList = p.nameList(p.name())
   756  	if p.gotAssign() {
   757  		d.Values = p.exprList()
   758  	} else {
   759  		d.Type = p.type_()
   760  		if p.gotAssign() {
   761  			d.Values = p.exprList()
   762  		}
   763  	}
   764  
   765  	return d
   766  }
   767  
   768  // FunctionDecl = "func" FunctionName [ TypeParams ] ( Function | Signature ) .
   769  // FunctionName = identifier .
   770  // Function     = Signature FunctionBody .
   771  // MethodDecl   = "func" Receiver MethodName ( Function | Signature ) .
   772  // Receiver     = Parameters .
   773  func (p *parser) funcDeclOrNil() *FuncDecl {
   774  	if trace {
   775  		defer p.trace("funcDecl")()
   776  	}
   777  
   778  	f := new(FuncDecl)
   779  	f.pos = p.pos()
   780  	f.Pragma = p.takePragma()
   781  
   782  	var context string
   783  	if p.got(_Lparen) {
   784  		context = "method"
   785  		rcvr := p.paramList(nil, nil, _Rparen, false)
   786  		switch len(rcvr) {
   787  		case 0:
   788  			p.error("method has no receiver")
   789  		default:
   790  			p.error("method has multiple receivers")
   791  			fallthrough
   792  		case 1:
   793  			f.Recv = rcvr[0]
   794  		}
   795  	}
   796  
   797  	if p.tok == _Name {
   798  		f.Name = p.name()
   799  		f.TParamList, f.Type = p.funcType(context)
   800  	} else {
   801  		f.Name = NewName(p.pos(), "_")
   802  		f.Type = new(FuncType)
   803  		f.Type.pos = p.pos()
   804  		msg := "expected name or ("
   805  		if context != "" {
   806  			msg = "expected name"
   807  		}
   808  		p.syntaxError(msg)
   809  		p.advance(_Lbrace, _Semi)
   810  	}
   811  
   812  	if p.tok == _Lbrace {
   813  		f.Body = p.funcBody()
   814  	}
   815  
   816  	return f
   817  }
   818  
   819  func (p *parser) funcBody() *BlockStmt {
   820  	p.fnest++
   821  	errcnt := p.errcnt
   822  	body := p.blockStmt("")
   823  	p.fnest--
   824  
   825  	// Don't check branches if there were syntax errors in the function
   826  	// as it may lead to spurious errors (e.g., see test/switch2.go) or
   827  	// possibly crashes due to incomplete syntax trees.
   828  	if p.mode&CheckBranches != 0 && errcnt == p.errcnt {
   829  		checkBranches(body, p.errh)
   830  	}
   831  
   832  	return body
   833  }
   834  
   835  // ----------------------------------------------------------------------------
   836  // Expressions
   837  
   838  func (p *parser) expr() Expr {
   839  	if trace {
   840  		defer p.trace("expr")()
   841  	}
   842  
   843  	return p.binaryExpr(nil, 0)
   844  }
   845  
   846  // Expression = UnaryExpr | Expression binary_op Expression .
   847  func (p *parser) binaryExpr(x Expr, prec int) Expr {
   848  	// don't trace binaryExpr - only leads to overly nested trace output
   849  
   850  	if x == nil {
   851  		x = p.unaryExpr()
   852  	}
   853  	for (p.tok == _Operator || p.tok == _Star) && p.prec > prec {
   854  		t := new(Operation)
   855  		t.pos = p.pos()
   856  		t.Op = p.op
   857  		tprec := p.prec
   858  		p.next()
   859  		t.X = x
   860  		t.Y = p.binaryExpr(nil, tprec)
   861  		x = t
   862  	}
   863  	return x
   864  }
   865  
   866  // UnaryExpr = PrimaryExpr | unary_op UnaryExpr .
   867  func (p *parser) unaryExpr() Expr {
   868  	if trace {
   869  		defer p.trace("unaryExpr")()
   870  	}
   871  
   872  	switch p.tok {
   873  	case _Operator, _Star:
   874  		switch p.op {
   875  		case Mul, Add, Sub, Not, Xor, Tilde:
   876  			x := new(Operation)
   877  			x.pos = p.pos()
   878  			x.Op = p.op
   879  			p.next()
   880  			x.X = p.unaryExpr()
   881  			return x
   882  
   883  		case And:
   884  			x := new(Operation)
   885  			x.pos = p.pos()
   886  			x.Op = And
   887  			p.next()
   888  			// unaryExpr may have returned a parenthesized composite literal
   889  			// (see comment in operand) - remove parentheses if any
   890  			x.X = Unparen(p.unaryExpr())
   891  			return x
   892  		}
   893  
   894  	case _Arrow:
   895  		// receive op (<-x) or receive-only channel (<-chan E)
   896  		pos := p.pos()
   897  		p.next()
   898  
   899  		// If the next token is _Chan we still don't know if it is
   900  		// a channel (<-chan int) or a receive op (<-chan int(ch)).
   901  		// We only know once we have found the end of the unaryExpr.
   902  
   903  		x := p.unaryExpr()
   904  
   905  		// There are two cases:
   906  		//
   907  		//   <-chan...  => <-x is a channel type
   908  		//   <-x        => <-x is a receive operation
   909  		//
   910  		// In the first case, <- must be re-associated with
   911  		// the channel type parsed already:
   912  		//
   913  		//   <-(chan E)   =>  (<-chan E)
   914  		//   <-(chan<-E)  =>  (<-chan (<-E))
   915  
   916  		if _, ok := x.(*ChanType); ok {
   917  			// x is a channel type => re-associate <-
   918  			dir := SendOnly
   919  			t := x
   920  			for dir == SendOnly {
   921  				c, ok := t.(*ChanType)
   922  				if !ok {
   923  					break
   924  				}
   925  				dir = c.Dir
   926  				if dir == RecvOnly {
   927  					// t is type <-chan E but <-<-chan E is not permitted
   928  					// (report same error as for "type _ <-<-chan E")
   929  					p.syntaxError("unexpected <-, expected chan")
   930  					// already progressed, no need to advance
   931  				}
   932  				c.Dir = RecvOnly
   933  				t = c.Elem
   934  			}
   935  			if dir == SendOnly {
   936  				// channel dir is <- but channel element E is not a channel
   937  				// (report same error as for "type _ <-chan<-E")
   938  				p.syntaxError(fmt.Sprintf("unexpected %s, expected chan", String(t)))
   939  				// already progressed, no need to advance
   940  			}
   941  			return x
   942  		}
   943  
   944  		// x is not a channel type => we have a receive op
   945  		o := new(Operation)
   946  		o.pos = pos
   947  		o.Op = Recv
   948  		o.X = x
   949  		return o
   950  	}
   951  
   952  	// TODO(mdempsky): We need parens here so we can report an
   953  	// error for "(x) := true". It should be possible to detect
   954  	// and reject that more efficiently though.
   955  	return p.pexpr(nil, true)
   956  }
   957  
   958  // callStmt parses call-like statements that can be preceded by 'defer' and 'go'.
   959  func (p *parser) callStmt() *CallStmt {
   960  	if trace {
   961  		defer p.trace("callStmt")()
   962  	}
   963  
   964  	s := new(CallStmt)
   965  	s.pos = p.pos()
   966  	s.Tok = p.tok // _Defer or _Go
   967  	p.next()
   968  
   969  	x := p.pexpr(nil, p.tok == _Lparen) // keep_parens so we can report error below
   970  	if t := Unparen(x); t != x {
   971  		p.errorAt(x.Pos(), fmt.Sprintf("expression in %s must not be parenthesized", s.Tok))
   972  		// already progressed, no need to advance
   973  		x = t
   974  	}
   975  
   976  	s.Call = x
   977  	return s
   978  }
   979  
   980  // Operand     = Literal | OperandName | MethodExpr | "(" Expression ")" .
   981  // Literal     = BasicLit | CompositeLit | FunctionLit .
   982  // BasicLit    = int_lit | float_lit | imaginary_lit | rune_lit | string_lit .
   983  // OperandName = identifier | QualifiedIdent.
   984  func (p *parser) operand(keep_parens bool) Expr {
   985  	if trace {
   986  		defer p.trace("operand " + p.tok.String())()
   987  	}
   988  
   989  	switch p.tok {
   990  	case _Name:
   991  		return p.name()
   992  
   993  	case _Literal:
   994  		return p.oliteral()
   995  
   996  	case _Lparen:
   997  		pos := p.pos()
   998  		p.next()
   999  		p.xnest++
  1000  		x := p.expr()
  1001  		p.xnest--
  1002  		p.want(_Rparen)
  1003  
  1004  		// Optimization: Record presence of ()'s only where needed
  1005  		// for error reporting. Don't bother in other cases; it is
  1006  		// just a waste of memory and time.
  1007  		//
  1008  		// Parentheses are not permitted around T in a composite
  1009  		// literal T{}. If the next token is a {, assume x is a
  1010  		// composite literal type T (it may not be, { could be
  1011  		// the opening brace of a block, but we don't know yet).
  1012  		if p.tok == _Lbrace {
  1013  			keep_parens = true
  1014  		}
  1015  
  1016  		// Parentheses are also not permitted around the expression
  1017  		// in a go/defer statement. In that case, operand is called
  1018  		// with keep_parens set.
  1019  		if keep_parens {
  1020  			px := new(ParenExpr)
  1021  			px.pos = pos
  1022  			px.X = x
  1023  			x = px
  1024  		}
  1025  		return x
  1026  
  1027  	case _Func:
  1028  		pos := p.pos()
  1029  		p.next()
  1030  		_, ftyp := p.funcType("function type")
  1031  		if p.tok == _Lbrace {
  1032  			p.xnest++
  1033  
  1034  			f := new(FuncLit)
  1035  			f.pos = pos
  1036  			f.Type = ftyp
  1037  			f.Body = p.funcBody()
  1038  
  1039  			p.xnest--
  1040  			return f
  1041  		}
  1042  		return ftyp
  1043  
  1044  	case _Lbrack, _Chan, _Map, _Struct, _Interface:
  1045  		return p.type_() // othertype
  1046  
  1047  	default:
  1048  		x := p.badExpr()
  1049  		p.syntaxError("expected expression")
  1050  		p.advance(_Rparen, _Rbrack, _Rbrace)
  1051  		return x
  1052  	}
  1053  
  1054  	// Syntactically, composite literals are operands. Because a complit
  1055  	// type may be a qualified identifier which is handled by pexpr
  1056  	// (together with selector expressions), complits are parsed there
  1057  	// as well (operand is only called from pexpr).
  1058  }
  1059  
  1060  // pexpr parses a PrimaryExpr.
  1061  //
  1062  //	PrimaryExpr =
  1063  //		Operand |
  1064  //		Conversion |
  1065  //		PrimaryExpr Selector |
  1066  //		PrimaryExpr Index |
  1067  //		PrimaryExpr Slice |
  1068  //		PrimaryExpr TypeAssertion |
  1069  //		PrimaryExpr Arguments .
  1070  //
  1071  //	Selector       = "." identifier .
  1072  //	Index          = "[" Expression "]" .
  1073  //	Slice          = "[" ( [ Expression ] ":" [ Expression ] ) |
  1074  //	                     ( [ Expression ] ":" Expression ":" Expression )
  1075  //	                 "]" .
  1076  //	TypeAssertion  = "." "(" Type ")" .
  1077  //	Arguments      = "(" [ ( ExpressionList | Type [ "," ExpressionList ] ) [ "..." ] [ "," ] ] ")" .
  1078  func (p *parser) pexpr(x Expr, keep_parens bool) Expr {
  1079  	if trace {
  1080  		defer p.trace("pexpr")()
  1081  	}
  1082  
  1083  	if x == nil {
  1084  		x = p.operand(keep_parens)
  1085  	}
  1086  
  1087  loop:
  1088  	for {
  1089  		pos := p.pos()
  1090  		switch p.tok {
  1091  		case _Dot:
  1092  			p.next()
  1093  			switch p.tok {
  1094  			case _Name:
  1095  				// pexpr '.' sym
  1096  				t := new(SelectorExpr)
  1097  				t.pos = pos
  1098  				t.X = x
  1099  				t.Sel = p.name()
  1100  				x = t
  1101  
  1102  			case _Lparen:
  1103  				p.next()
  1104  				if p.got(_Type) {
  1105  					t := new(TypeSwitchGuard)
  1106  					// t.Lhs is filled in by parser.simpleStmt
  1107  					t.pos = pos
  1108  					t.X = x
  1109  					x = t
  1110  				} else {
  1111  					t := new(AssertExpr)
  1112  					t.pos = pos
  1113  					t.X = x
  1114  					t.Type = p.type_()
  1115  					x = t
  1116  				}
  1117  				p.want(_Rparen)
  1118  
  1119  			default:
  1120  				p.syntaxError("expected name or (")
  1121  				p.advance(_Semi, _Rparen)
  1122  			}
  1123  
  1124  		case _Lbrack:
  1125  			p.next()
  1126  
  1127  			var i Expr
  1128  			if p.tok != _Colon {
  1129  				var comma bool
  1130  				if p.tok == _Rbrack {
  1131  					// invalid empty instance, slice or index expression; accept but complain
  1132  					p.syntaxError("expected operand")
  1133  					i = p.badExpr()
  1134  				} else {
  1135  					i, comma = p.typeList(false)
  1136  				}
  1137  				if comma || p.tok == _Rbrack {
  1138  					p.want(_Rbrack)
  1139  					// x[], x[i,] or x[i, j, ...]
  1140  					t := new(IndexExpr)
  1141  					t.pos = pos
  1142  					t.X = x
  1143  					t.Index = i
  1144  					x = t
  1145  					break
  1146  				}
  1147  			}
  1148  
  1149  			// x[i:...
  1150  			// For better error message, don't simply use p.want(_Colon) here (go.dev/issue/47704).
  1151  			if !p.got(_Colon) {
  1152  				p.syntaxError("expected comma, : or ]")
  1153  				p.advance(_Comma, _Colon, _Rbrack)
  1154  			}
  1155  			p.xnest++
  1156  			t := new(SliceExpr)
  1157  			t.pos = pos
  1158  			t.X = x
  1159  			t.Index[0] = i
  1160  			if p.tok != _Colon && p.tok != _Rbrack {
  1161  				// x[i:j...
  1162  				t.Index[1] = p.expr()
  1163  			}
  1164  			if p.tok == _Colon {
  1165  				t.Full = true
  1166  				// x[i:j:...]
  1167  				if t.Index[1] == nil {
  1168  					p.error("middle index required in 3-index slice")
  1169  					t.Index[1] = p.badExpr()
  1170  				}
  1171  				p.next()
  1172  				if p.tok != _Rbrack {
  1173  					// x[i:j:k...
  1174  					t.Index[2] = p.expr()
  1175  				} else {
  1176  					p.error("final index required in 3-index slice")
  1177  					t.Index[2] = p.badExpr()
  1178  				}
  1179  			}
  1180  			p.xnest--
  1181  			p.want(_Rbrack)
  1182  			x = t
  1183  
  1184  		case _Lparen:
  1185  			t := new(CallExpr)
  1186  			t.pos = pos
  1187  			p.next()
  1188  			t.Fun = x
  1189  			t.ArgList, t.HasDots = p.argList()
  1190  			x = t
  1191  
  1192  		case _Lbrace:
  1193  			// operand may have returned a parenthesized complit
  1194  			// type; accept it but complain if we have a complit
  1195  			t := Unparen(x)
  1196  			// determine if '{' belongs to a composite literal or a block statement
  1197  			complit_ok := false
  1198  			switch t.(type) {
  1199  			case *Name, *SelectorExpr:
  1200  				if p.xnest >= 0 {
  1201  					// x is possibly a composite literal type
  1202  					complit_ok = true
  1203  				}
  1204  			case *IndexExpr:
  1205  				if p.xnest >= 0 && !isValue(t) {
  1206  					// x is possibly a composite literal type
  1207  					complit_ok = true
  1208  				}
  1209  			case *ArrayType, *SliceType, *StructType, *MapType:
  1210  				// x is a comptype
  1211  				complit_ok = true
  1212  			}
  1213  			if !complit_ok {
  1214  				break loop
  1215  			}
  1216  			if t != x {
  1217  				p.syntaxError("cannot parenthesize type in composite literal")
  1218  				// already progressed, no need to advance
  1219  			}
  1220  			n := p.complitexpr()
  1221  			n.Type = x
  1222  			x = n
  1223  
  1224  		default:
  1225  			break loop
  1226  		}
  1227  	}
  1228  
  1229  	return x
  1230  }
  1231  
  1232  // isValue reports whether x syntactically must be a value (and not a type) expression.
  1233  func isValue(x Expr) bool {
  1234  	switch x := x.(type) {
  1235  	case *BasicLit, *CompositeLit, *FuncLit, *SliceExpr, *AssertExpr, *TypeSwitchGuard, *CallExpr:
  1236  		return true
  1237  	case *Operation:
  1238  		return x.Op != Mul || x.Y != nil // *T may be a type
  1239  	case *ParenExpr:
  1240  		return isValue(x.X)
  1241  	case *IndexExpr:
  1242  		return isValue(x.X) || isValue(x.Index)
  1243  	}
  1244  	return false
  1245  }
  1246  
  1247  // Element = Expression | LiteralValue .
  1248  func (p *parser) bare_complitexpr() Expr {
  1249  	if trace {
  1250  		defer p.trace("bare_complitexpr")()
  1251  	}
  1252  
  1253  	if p.tok == _Lbrace {
  1254  		// '{' start_complit braced_keyval_list '}'
  1255  		return p.complitexpr()
  1256  	}
  1257  
  1258  	return p.expr()
  1259  }
  1260  
  1261  // LiteralValue = "{" [ ElementList [ "," ] ] "}" .
  1262  func (p *parser) complitexpr() *CompositeLit {
  1263  	if trace {
  1264  		defer p.trace("complitexpr")()
  1265  	}
  1266  
  1267  	x := new(CompositeLit)
  1268  	x.pos = p.pos()
  1269  
  1270  	p.xnest++
  1271  	p.want(_Lbrace)
  1272  	x.Rbrace = p.list("composite literal", _Comma, _Rbrace, func() bool {
  1273  		// value
  1274  		e := p.bare_complitexpr()
  1275  		if p.tok == _Colon {
  1276  			// key ':' value
  1277  			l := new(KeyValueExpr)
  1278  			l.pos = p.pos()
  1279  			p.next()
  1280  			l.Key = e
  1281  			l.Value = p.bare_complitexpr()
  1282  			e = l
  1283  			x.NKeys++
  1284  		}
  1285  		x.ElemList = append(x.ElemList, e)
  1286  		return false
  1287  	})
  1288  	p.xnest--
  1289  
  1290  	return x
  1291  }
  1292  
  1293  // ----------------------------------------------------------------------------
  1294  // Types
  1295  
  1296  func (p *parser) type_() Expr {
  1297  	if trace {
  1298  		defer p.trace("type_")()
  1299  	}
  1300  
  1301  	typ := p.typeOrNil()
  1302  	if typ == nil {
  1303  		typ = p.badExpr()
  1304  		p.syntaxError("expected type")
  1305  		p.advance(_Comma, _Colon, _Semi, _Rparen, _Rbrack, _Rbrace)
  1306  	}
  1307  
  1308  	return typ
  1309  }
  1310  
  1311  func newIndirect(pos Pos, typ Expr) Expr {
  1312  	o := new(Operation)
  1313  	o.pos = pos
  1314  	o.Op = Mul
  1315  	o.X = typ
  1316  	return o
  1317  }
  1318  
  1319  // typeOrNil is like type_ but it returns nil if there was no type
  1320  // instead of reporting an error.
  1321  //
  1322  //	Type     = TypeName | TypeLit | "(" Type ")" .
  1323  //	TypeName = identifier | QualifiedIdent .
  1324  //	TypeLit  = ArrayType | StructType | PointerType | FunctionType | InterfaceType |
  1325  //		      SliceType | MapType | Channel_Type .
  1326  func (p *parser) typeOrNil() Expr {
  1327  	if trace {
  1328  		defer p.trace("typeOrNil")()
  1329  	}
  1330  
  1331  	pos := p.pos()
  1332  	switch p.tok {
  1333  	case _Star:
  1334  		// ptrtype
  1335  		p.next()
  1336  		return newIndirect(pos, p.type_())
  1337  
  1338  	case _Arrow:
  1339  		// recvchantype
  1340  		p.next()
  1341  		p.want(_Chan)
  1342  		t := new(ChanType)
  1343  		t.pos = pos
  1344  		t.Dir = RecvOnly
  1345  		t.Elem = p.chanElem()
  1346  		return t
  1347  
  1348  	case _Func:
  1349  		// fntype
  1350  		p.next()
  1351  		_, t := p.funcType("function type")
  1352  		return t
  1353  
  1354  	case _Lbrack:
  1355  		// '[' oexpr ']' ntype
  1356  		// '[' _DotDotDot ']' ntype
  1357  		p.next()
  1358  		if p.got(_Rbrack) {
  1359  			return p.sliceType(pos)
  1360  		}
  1361  		return p.arrayType(pos, nil)
  1362  
  1363  	case _Chan:
  1364  		// _Chan non_recvchantype
  1365  		// _Chan _Comm ntype
  1366  		p.next()
  1367  		t := new(ChanType)
  1368  		t.pos = pos
  1369  		if p.got(_Arrow) {
  1370  			t.Dir = SendOnly
  1371  		}
  1372  		t.Elem = p.chanElem()
  1373  		return t
  1374  
  1375  	case _Map:
  1376  		// _Map '[' ntype ']' ntype
  1377  		p.next()
  1378  		p.want(_Lbrack)
  1379  		t := new(MapType)
  1380  		t.pos = pos
  1381  		t.Key = p.type_()
  1382  		p.want(_Rbrack)
  1383  		t.Value = p.type_()
  1384  		return t
  1385  
  1386  	case _Struct:
  1387  		return p.structType()
  1388  
  1389  	case _Interface:
  1390  		return p.interfaceType()
  1391  
  1392  	case _Name:
  1393  		return p.qualifiedName(nil)
  1394  
  1395  	case _Lparen:
  1396  		p.next()
  1397  		t := p.type_()
  1398  		p.want(_Rparen)
  1399  		return t
  1400  	}
  1401  
  1402  	return nil
  1403  }
  1404  
  1405  func (p *parser) typeInstance(typ Expr) Expr {
  1406  	if trace {
  1407  		defer p.trace("typeInstance")()
  1408  	}
  1409  
  1410  	pos := p.pos()
  1411  	p.want(_Lbrack)
  1412  	x := new(IndexExpr)
  1413  	x.pos = pos
  1414  	x.X = typ
  1415  	if p.tok == _Rbrack {
  1416  		p.syntaxError("expected type argument list")
  1417  		x.Index = p.badExpr()
  1418  	} else {
  1419  		x.Index, _ = p.typeList(true)
  1420  	}
  1421  	p.want(_Rbrack)
  1422  	return x
  1423  }
  1424  
  1425  // If context != "", type parameters are not permitted.
  1426  func (p *parser) funcType(context string) ([]*Field, *FuncType) {
  1427  	if trace {
  1428  		defer p.trace("funcType")()
  1429  	}
  1430  
  1431  	typ := new(FuncType)
  1432  	typ.pos = p.pos()
  1433  
  1434  	var tparamList []*Field
  1435  	if p.got(_Lbrack) {
  1436  		if context != "" {
  1437  			// accept but complain
  1438  			p.syntaxErrorAt(typ.pos, context+" must have no type parameters")
  1439  		}
  1440  		if p.tok == _Rbrack {
  1441  			p.syntaxError("empty type parameter list")
  1442  			p.next()
  1443  		} else {
  1444  			tparamList = p.paramList(nil, nil, _Rbrack, true)
  1445  		}
  1446  	}
  1447  
  1448  	p.want(_Lparen)
  1449  	typ.ParamList = p.paramList(nil, nil, _Rparen, false)
  1450  	typ.ResultList = p.funcResult()
  1451  
  1452  	return tparamList, typ
  1453  }
  1454  
  1455  // "[" has already been consumed, and pos is its position.
  1456  // If len != nil it is the already consumed array length.
  1457  func (p *parser) arrayType(pos Pos, len Expr) Expr {
  1458  	if trace {
  1459  		defer p.trace("arrayType")()
  1460  	}
  1461  
  1462  	if len == nil && !p.got(_DotDotDot) {
  1463  		p.xnest++
  1464  		len = p.expr()
  1465  		p.xnest--
  1466  	}
  1467  	if p.tok == _Comma {
  1468  		// Trailing commas are accepted in type parameter
  1469  		// lists but not in array type declarations.
  1470  		// Accept for better error handling but complain.
  1471  		p.syntaxError("unexpected comma; expected ]")
  1472  		p.next()
  1473  	}
  1474  	p.want(_Rbrack)
  1475  	t := new(ArrayType)
  1476  	t.pos = pos
  1477  	t.Len = len
  1478  	t.Elem = p.type_()
  1479  	return t
  1480  }
  1481  
  1482  // "[" and "]" have already been consumed, and pos is the position of "[".
  1483  func (p *parser) sliceType(pos Pos) Expr {
  1484  	t := new(SliceType)
  1485  	t.pos = pos
  1486  	t.Elem = p.type_()
  1487  	return t
  1488  }
  1489  
  1490  func (p *parser) chanElem() Expr {
  1491  	if trace {
  1492  		defer p.trace("chanElem")()
  1493  	}
  1494  
  1495  	typ := p.typeOrNil()
  1496  	if typ == nil {
  1497  		typ = p.badExpr()
  1498  		p.syntaxError("missing channel element type")
  1499  		// assume element type is simply absent - don't advance
  1500  	}
  1501  
  1502  	return typ
  1503  }
  1504  
  1505  // StructType = "struct" "{" { FieldDecl ";" } "}" .
  1506  func (p *parser) structType() *StructType {
  1507  	if trace {
  1508  		defer p.trace("structType")()
  1509  	}
  1510  
  1511  	typ := new(StructType)
  1512  	typ.pos = p.pos()
  1513  
  1514  	p.want(_Struct)
  1515  	p.want(_Lbrace)
  1516  	p.list("struct type", _Semi, _Rbrace, func() bool {
  1517  		p.fieldDecl(typ)
  1518  		return false
  1519  	})
  1520  
  1521  	return typ
  1522  }
  1523  
  1524  // InterfaceType = "interface" "{" { ( MethodDecl | EmbeddedElem ) ";" } "}" .
  1525  func (p *parser) interfaceType() *InterfaceType {
  1526  	if trace {
  1527  		defer p.trace("interfaceType")()
  1528  	}
  1529  
  1530  	typ := new(InterfaceType)
  1531  	typ.pos = p.pos()
  1532  
  1533  	p.want(_Interface)
  1534  	p.want(_Lbrace)
  1535  	p.list("interface type", _Semi, _Rbrace, func() bool {
  1536  		var f *Field
  1537  		if p.tok == _Name {
  1538  			f = p.methodDecl()
  1539  		}
  1540  		if f == nil || f.Name == nil {
  1541  			f = p.embeddedElem(f)
  1542  		}
  1543  		typ.MethodList = append(typ.MethodList, f)
  1544  		return false
  1545  	})
  1546  
  1547  	return typ
  1548  }
  1549  
  1550  // Result = Parameters | Type .
  1551  func (p *parser) funcResult() []*Field {
  1552  	if trace {
  1553  		defer p.trace("funcResult")()
  1554  	}
  1555  
  1556  	if p.got(_Lparen) {
  1557  		return p.paramList(nil, nil, _Rparen, false)
  1558  	}
  1559  
  1560  	pos := p.pos()
  1561  	if typ := p.typeOrNil(); typ != nil {
  1562  		f := new(Field)
  1563  		f.pos = pos
  1564  		f.Type = typ
  1565  		return []*Field{f}
  1566  	}
  1567  
  1568  	return nil
  1569  }
  1570  
  1571  func (p *parser) addField(styp *StructType, pos Pos, name *Name, typ Expr, tag *BasicLit) {
  1572  	if tag != nil {
  1573  		for i := len(styp.FieldList) - len(styp.TagList); i > 0; i-- {
  1574  			styp.TagList = append(styp.TagList, nil)
  1575  		}
  1576  		styp.TagList = append(styp.TagList, tag)
  1577  	}
  1578  
  1579  	f := new(Field)
  1580  	f.pos = pos
  1581  	f.Name = name
  1582  	f.Type = typ
  1583  	styp.FieldList = append(styp.FieldList, f)
  1584  
  1585  	if debug && tag != nil && len(styp.FieldList) != len(styp.TagList) {
  1586  		panic("inconsistent struct field list")
  1587  	}
  1588  }
  1589  
  1590  // FieldDecl      = (IdentifierList Type | AnonymousField) [ Tag ] .
  1591  // AnonymousField = [ "*" ] TypeName .
  1592  // Tag            = string_lit .
  1593  func (p *parser) fieldDecl(styp *StructType) {
  1594  	if trace {
  1595  		defer p.trace("fieldDecl")()
  1596  	}
  1597  
  1598  	pos := p.pos()
  1599  	switch p.tok {
  1600  	case _Name:
  1601  		name := p.name()
  1602  		if p.tok == _Dot || p.tok == _Literal || p.tok == _Semi || p.tok == _Rbrace {
  1603  			// embedded type
  1604  			typ := p.qualifiedName(name)
  1605  			tag := p.oliteral()
  1606  			p.addField(styp, pos, nil, typ, tag)
  1607  			break
  1608  		}
  1609  
  1610  		// name1, name2, ... Type [ tag ]
  1611  		names := p.nameList(name)
  1612  		var typ Expr
  1613  
  1614  		// Careful dance: We don't know if we have an embedded instantiated
  1615  		// type T[P1, P2, ...] or a field T of array/slice type [P]E or []E.
  1616  		if len(names) == 1 && p.tok == _Lbrack {
  1617  			typ = p.arrayOrTArgs()
  1618  			if typ, ok := typ.(*IndexExpr); ok {
  1619  				// embedded type T[P1, P2, ...]
  1620  				typ.X = name // name == names[0]
  1621  				tag := p.oliteral()
  1622  				p.addField(styp, pos, nil, typ, tag)
  1623  				break
  1624  			}
  1625  		} else {
  1626  			// T P
  1627  			typ = p.type_()
  1628  		}
  1629  
  1630  		tag := p.oliteral()
  1631  
  1632  		for _, name := range names {
  1633  			p.addField(styp, name.Pos(), name, typ, tag)
  1634  		}
  1635  
  1636  	case _Star:
  1637  		p.next()
  1638  		var typ Expr
  1639  		if p.tok == _Lparen {
  1640  			// *(T)
  1641  			p.syntaxError("cannot parenthesize embedded type")
  1642  			p.next()
  1643  			typ = p.qualifiedName(nil)
  1644  			p.got(_Rparen) // no need to complain if missing
  1645  		} else {
  1646  			// *T
  1647  			typ = p.qualifiedName(nil)
  1648  		}
  1649  		tag := p.oliteral()
  1650  		p.addField(styp, pos, nil, newIndirect(pos, typ), tag)
  1651  
  1652  	case _Lparen:
  1653  		p.syntaxError("cannot parenthesize embedded type")
  1654  		p.next()
  1655  		var typ Expr
  1656  		if p.tok == _Star {
  1657  			// (*T)
  1658  			pos := p.pos()
  1659  			p.next()
  1660  			typ = newIndirect(pos, p.qualifiedName(nil))
  1661  		} else {
  1662  			// (T)
  1663  			typ = p.qualifiedName(nil)
  1664  		}
  1665  		p.got(_Rparen) // no need to complain if missing
  1666  		tag := p.oliteral()
  1667  		p.addField(styp, pos, nil, typ, tag)
  1668  
  1669  	default:
  1670  		p.syntaxError("expected field name or embedded type")
  1671  		p.advance(_Semi, _Rbrace)
  1672  	}
  1673  }
  1674  
  1675  func (p *parser) arrayOrTArgs() Expr {
  1676  	if trace {
  1677  		defer p.trace("arrayOrTArgs")()
  1678  	}
  1679  
  1680  	pos := p.pos()
  1681  	p.want(_Lbrack)
  1682  	if p.got(_Rbrack) {
  1683  		return p.sliceType(pos)
  1684  	}
  1685  
  1686  	// x [n]E or x[n,], x[n1, n2], ...
  1687  	n, comma := p.typeList(false)
  1688  	p.want(_Rbrack)
  1689  	if !comma {
  1690  		if elem := p.typeOrNil(); elem != nil {
  1691  			// x [n]E
  1692  			t := new(ArrayType)
  1693  			t.pos = pos
  1694  			t.Len = n
  1695  			t.Elem = elem
  1696  			return t
  1697  		}
  1698  	}
  1699  
  1700  	// x[n,], x[n1, n2], ...
  1701  	t := new(IndexExpr)
  1702  	t.pos = pos
  1703  	// t.X will be filled in by caller
  1704  	t.Index = n
  1705  	return t
  1706  }
  1707  
  1708  func (p *parser) oliteral() *BasicLit {
  1709  	if p.tok == _Literal {
  1710  		b := new(BasicLit)
  1711  		b.pos = p.pos()
  1712  		b.Value = p.lit
  1713  		b.Kind = p.kind
  1714  		b.Bad = p.bad
  1715  		p.next()
  1716  		return b
  1717  	}
  1718  	return nil
  1719  }
  1720  
  1721  // MethodSpec        = MethodName Signature | InterfaceTypeName .
  1722  // MethodName        = identifier .
  1723  // InterfaceTypeName = TypeName .
  1724  func (p *parser) methodDecl() *Field {
  1725  	if trace {
  1726  		defer p.trace("methodDecl")()
  1727  	}
  1728  
  1729  	f := new(Field)
  1730  	f.pos = p.pos()
  1731  	name := p.name()
  1732  
  1733  	const context = "interface method"
  1734  
  1735  	switch p.tok {
  1736  	case _Lparen:
  1737  		// method
  1738  		f.Name = name
  1739  		_, f.Type = p.funcType(context)
  1740  
  1741  	case _Lbrack:
  1742  		// Careful dance: We don't know if we have a generic method m[T C](x T)
  1743  		// or an embedded instantiated type T[P1, P2] (we accept generic methods
  1744  		// for generality and robustness of parsing but complain with an error).
  1745  		pos := p.pos()
  1746  		p.next()
  1747  
  1748  		// Empty type parameter or argument lists are not permitted.
  1749  		// Treat as if [] were absent.
  1750  		if p.tok == _Rbrack {
  1751  			// name[]
  1752  			pos := p.pos()
  1753  			p.next()
  1754  			if p.tok == _Lparen {
  1755  				// name[](
  1756  				p.errorAt(pos, "empty type parameter list")
  1757  				f.Name = name
  1758  				_, f.Type = p.funcType(context)
  1759  			} else {
  1760  				p.errorAt(pos, "empty type argument list")
  1761  				f.Type = name
  1762  			}
  1763  			break
  1764  		}
  1765  
  1766  		// A type argument list looks like a parameter list with only
  1767  		// types. Parse a parameter list and decide afterwards.
  1768  		list := p.paramList(nil, nil, _Rbrack, false)
  1769  		if len(list) == 0 {
  1770  			// The type parameter list is not [] but we got nothing
  1771  			// due to other errors (reported by paramList). Treat
  1772  			// as if [] were absent.
  1773  			if p.tok == _Lparen {
  1774  				f.Name = name
  1775  				_, f.Type = p.funcType(context)
  1776  			} else {
  1777  				f.Type = name
  1778  			}
  1779  			break
  1780  		}
  1781  
  1782  		// len(list) > 0
  1783  		if list[0].Name != nil {
  1784  			// generic method
  1785  			f.Name = name
  1786  			_, f.Type = p.funcType(context)
  1787  			p.errorAt(pos, "interface method must have no type parameters")
  1788  			break
  1789  		}
  1790  
  1791  		// embedded instantiated type
  1792  		t := new(IndexExpr)
  1793  		t.pos = pos
  1794  		t.X = name
  1795  		if len(list) == 1 {
  1796  			t.Index = list[0].Type
  1797  		} else {
  1798  			// len(list) > 1
  1799  			l := new(ListExpr)
  1800  			l.pos = list[0].Pos()
  1801  			l.ElemList = make([]Expr, len(list))
  1802  			for i := range list {
  1803  				l.ElemList[i] = list[i].Type
  1804  			}
  1805  			t.Index = l
  1806  		}
  1807  		f.Type = t
  1808  
  1809  	default:
  1810  		// embedded type
  1811  		f.Type = p.qualifiedName(name)
  1812  	}
  1813  
  1814  	return f
  1815  }
  1816  
  1817  // EmbeddedElem = MethodSpec | EmbeddedTerm { "|" EmbeddedTerm } .
  1818  func (p *parser) embeddedElem(f *Field) *Field {
  1819  	if trace {
  1820  		defer p.trace("embeddedElem")()
  1821  	}
  1822  
  1823  	if f == nil {
  1824  		f = new(Field)
  1825  		f.pos = p.pos()
  1826  		f.Type = p.embeddedTerm()
  1827  	}
  1828  
  1829  	for p.tok == _Operator && p.op == Or {
  1830  		t := new(Operation)
  1831  		t.pos = p.pos()
  1832  		t.Op = Or
  1833  		p.next()
  1834  		t.X = f.Type
  1835  		t.Y = p.embeddedTerm()
  1836  		f.Type = t
  1837  	}
  1838  
  1839  	return f
  1840  }
  1841  
  1842  // EmbeddedTerm = [ "~" ] Type .
  1843  func (p *parser) embeddedTerm() Expr {
  1844  	if trace {
  1845  		defer p.trace("embeddedTerm")()
  1846  	}
  1847  
  1848  	if p.tok == _Operator && p.op == Tilde {
  1849  		t := new(Operation)
  1850  		t.pos = p.pos()
  1851  		t.Op = Tilde
  1852  		p.next()
  1853  		t.X = p.type_()
  1854  		return t
  1855  	}
  1856  
  1857  	t := p.typeOrNil()
  1858  	if t == nil {
  1859  		t = p.badExpr()
  1860  		p.syntaxError("expected ~ term or type")
  1861  		p.advance(_Operator, _Semi, _Rparen, _Rbrack, _Rbrace)
  1862  	}
  1863  
  1864  	return t
  1865  }
  1866  
  1867  // ParameterDecl = [ IdentifierList ] [ "..." ] Type .
  1868  func (p *parser) paramDeclOrNil(name *Name, follow token) *Field {
  1869  	if trace {
  1870  		defer p.trace("paramDeclOrNil")()
  1871  	}
  1872  
  1873  	// type set notation is ok in type parameter lists
  1874  	typeSetsOk := follow == _Rbrack
  1875  
  1876  	pos := p.pos()
  1877  	if name != nil {
  1878  		pos = name.pos
  1879  	} else if typeSetsOk && p.tok == _Operator && p.op == Tilde {
  1880  		// "~" ...
  1881  		return p.embeddedElem(nil)
  1882  	}
  1883  
  1884  	f := new(Field)
  1885  	f.pos = pos
  1886  
  1887  	if p.tok == _Name || name != nil {
  1888  		// name
  1889  		if name == nil {
  1890  			name = p.name()
  1891  		}
  1892  
  1893  		if p.tok == _Lbrack {
  1894  			// name "[" ...
  1895  			f.Type = p.arrayOrTArgs()
  1896  			if typ, ok := f.Type.(*IndexExpr); ok {
  1897  				// name "[" ... "]"
  1898  				typ.X = name
  1899  			} else {
  1900  				// name "[" n "]" E
  1901  				f.Name = name
  1902  			}
  1903  			if typeSetsOk && p.tok == _Operator && p.op == Or {
  1904  				// name "[" ... "]" "|" ...
  1905  				// name "[" n "]" E "|" ...
  1906  				f = p.embeddedElem(f)
  1907  			}
  1908  			return f
  1909  		}
  1910  
  1911  		if p.tok == _Dot {
  1912  			// name "." ...
  1913  			f.Type = p.qualifiedName(name)
  1914  			if typeSetsOk && p.tok == _Operator && p.op == Or {
  1915  				// name "." name "|" ...
  1916  				f = p.embeddedElem(f)
  1917  			}
  1918  			return f
  1919  		}
  1920  
  1921  		if typeSetsOk && p.tok == _Operator && p.op == Or {
  1922  			// name "|" ...
  1923  			f.Type = name
  1924  			return p.embeddedElem(f)
  1925  		}
  1926  
  1927  		f.Name = name
  1928  	}
  1929  
  1930  	if p.tok == _DotDotDot {
  1931  		// [name] "..." ...
  1932  		t := new(DotsType)
  1933  		t.pos = p.pos()
  1934  		p.next()
  1935  		t.Elem = p.typeOrNil()
  1936  		if t.Elem == nil {
  1937  			t.Elem = p.badExpr()
  1938  			p.syntaxError("... is missing type")
  1939  		}
  1940  		f.Type = t
  1941  		return f
  1942  	}
  1943  
  1944  	if typeSetsOk && p.tok == _Operator && p.op == Tilde {
  1945  		// [name] "~" ...
  1946  		f.Type = p.embeddedElem(nil).Type
  1947  		return f
  1948  	}
  1949  
  1950  	f.Type = p.typeOrNil()
  1951  	if typeSetsOk && p.tok == _Operator && p.op == Or && f.Type != nil {
  1952  		// [name] type "|"
  1953  		f = p.embeddedElem(f)
  1954  	}
  1955  	if f.Name != nil || f.Type != nil {
  1956  		return f
  1957  	}
  1958  
  1959  	p.syntaxError("expected " + tokstring(follow))
  1960  	p.advance(_Comma, follow)
  1961  	return nil
  1962  }
  1963  
  1964  // Parameters    = "(" [ ParameterList [ "," ] ] ")" .
  1965  // ParameterList = ParameterDecl { "," ParameterDecl } .
  1966  // "(" or "[" has already been consumed.
  1967  // If name != nil, it is the first name after "(" or "[".
  1968  // If typ != nil, name must be != nil, and (name, typ) is the first field in the list.
  1969  // In the result list, either all fields have a name, or no field has a name.
  1970  func (p *parser) paramList(name *Name, typ Expr, close token, requireNames bool) (list []*Field) {
  1971  	if trace {
  1972  		defer p.trace("paramList")()
  1973  	}
  1974  
  1975  	// p.list won't invoke its function argument if we're at the end of the
  1976  	// parameter list. If we have a complete field, handle this case here.
  1977  	if name != nil && typ != nil && p.tok == close {
  1978  		p.next()
  1979  		par := new(Field)
  1980  		par.pos = name.pos
  1981  		par.Name = name
  1982  		par.Type = typ
  1983  		return []*Field{par}
  1984  	}
  1985  
  1986  	var named int // number of parameters that have an explicit name and type
  1987  	var typed int // number of parameters that have an explicit type
  1988  	end := p.list("parameter list", _Comma, close, func() bool {
  1989  		var par *Field
  1990  		if typ != nil {
  1991  			if debug && name == nil {
  1992  				panic("initial type provided without name")
  1993  			}
  1994  			par = new(Field)
  1995  			par.pos = name.pos
  1996  			par.Name = name
  1997  			par.Type = typ
  1998  		} else {
  1999  			par = p.paramDeclOrNil(name, close)
  2000  		}
  2001  		name = nil // 1st name was consumed if present
  2002  		typ = nil  // 1st type was consumed if present
  2003  		if par != nil {
  2004  			if debug && par.Name == nil && par.Type == nil {
  2005  				panic("parameter without name or type")
  2006  			}
  2007  			if par.Name != nil && par.Type != nil {
  2008  				named++
  2009  			}
  2010  			if par.Type != nil {
  2011  				typed++
  2012  			}
  2013  			list = append(list, par)
  2014  		}
  2015  		return false
  2016  	})
  2017  
  2018  	if len(list) == 0 {
  2019  		return
  2020  	}
  2021  
  2022  	// distribute parameter types (len(list) > 0)
  2023  	if named == 0 && !requireNames {
  2024  		// all unnamed and we're not in a type parameter list => found names are named types
  2025  		for _, par := range list {
  2026  			if typ := par.Name; typ != nil {
  2027  				par.Type = typ
  2028  				par.Name = nil
  2029  			}
  2030  		}
  2031  	} else if named != len(list) {
  2032  		// some named or we're in a type parameter list => all must be named
  2033  		var errPos Pos // left-most error position (or unknown)
  2034  		var typ Expr   // current type (from right to left)
  2035  		for i := len(list) - 1; i >= 0; i-- {
  2036  			par := list[i]
  2037  			if par.Type != nil {
  2038  				typ = par.Type
  2039  				if par.Name == nil {
  2040  					errPos = StartPos(typ)
  2041  					par.Name = NewName(errPos, "_")
  2042  				}
  2043  			} else if typ != nil {
  2044  				par.Type = typ
  2045  			} else {
  2046  				// par.Type == nil && typ == nil => we only have a par.Name
  2047  				errPos = par.Name.Pos()
  2048  				t := p.badExpr()
  2049  				t.pos = errPos // correct position
  2050  				par.Type = t
  2051  			}
  2052  		}
  2053  		if errPos.IsKnown() {
  2054  			var msg string
  2055  			if requireNames {
  2056  				// Not all parameters are named because named != len(list).
  2057  				// If named == typed we must have parameters that have no types,
  2058  				// and they must be at the end of the parameter list, otherwise
  2059  				// the types would have been filled in by the right-to-left sweep
  2060  				// above and we wouldn't have an error. Since we are in a type
  2061  				// parameter list, the missing types are constraints.
  2062  				if named == typed {
  2063  					errPos = end // position error at closing ]
  2064  					msg = "missing type constraint"
  2065  				} else {
  2066  					msg = "missing type parameter name"
  2067  					// go.dev/issue/60812
  2068  					if len(list) == 1 {
  2069  						msg += " or invalid array length"
  2070  					}
  2071  				}
  2072  			} else {
  2073  				msg = "mixed named and unnamed parameters"
  2074  			}
  2075  			p.syntaxErrorAt(errPos, msg)
  2076  		}
  2077  	}
  2078  
  2079  	return
  2080  }
  2081  
  2082  func (p *parser) badExpr() *BadExpr {
  2083  	b := new(BadExpr)
  2084  	b.pos = p.pos()
  2085  	return b
  2086  }
  2087  
  2088  // ----------------------------------------------------------------------------
  2089  // Statements
  2090  
  2091  // SimpleStmt = EmptyStmt | ExpressionStmt | SendStmt | IncDecStmt | Assignment | ShortVarDecl .
  2092  func (p *parser) simpleStmt(lhs Expr, keyword token) SimpleStmt {
  2093  	if trace {
  2094  		defer p.trace("simpleStmt")()
  2095  	}
  2096  
  2097  	if keyword == _For && p.tok == _Range {
  2098  		// _Range expr
  2099  		if debug && lhs != nil {
  2100  			panic("invalid call of simpleStmt")
  2101  		}
  2102  		return p.newRangeClause(nil, false)
  2103  	}
  2104  
  2105  	if lhs == nil {
  2106  		lhs = p.exprList()
  2107  	}
  2108  
  2109  	if _, ok := lhs.(*ListExpr); !ok && p.tok != _Assign && p.tok != _Define {
  2110  		// expr
  2111  		pos := p.pos()
  2112  		switch p.tok {
  2113  		case _AssignOp:
  2114  			// lhs op= rhs
  2115  			op := p.op
  2116  			p.next()
  2117  			return p.newAssignStmt(pos, op, lhs, p.expr())
  2118  
  2119  		case _IncOp:
  2120  			// lhs++ or lhs--
  2121  			op := p.op
  2122  			p.next()
  2123  			return p.newAssignStmt(pos, op, lhs, nil)
  2124  
  2125  		case _Arrow:
  2126  			// lhs <- rhs
  2127  			s := new(SendStmt)
  2128  			s.pos = pos
  2129  			p.next()
  2130  			s.Chan = lhs
  2131  			s.Value = p.expr()
  2132  			return s
  2133  
  2134  		default:
  2135  			// expr
  2136  			s := new(ExprStmt)
  2137  			s.pos = lhs.Pos()
  2138  			s.X = lhs
  2139  			return s
  2140  		}
  2141  	}
  2142  
  2143  	// expr_list
  2144  	switch p.tok {
  2145  	case _Assign, _Define:
  2146  		pos := p.pos()
  2147  		var op Operator
  2148  		if p.tok == _Define {
  2149  			op = Def
  2150  		}
  2151  		p.next()
  2152  
  2153  		if keyword == _For && p.tok == _Range {
  2154  			// expr_list op= _Range expr
  2155  			return p.newRangeClause(lhs, op == Def)
  2156  		}
  2157  
  2158  		// expr_list op= expr_list
  2159  		rhs := p.exprList()
  2160  
  2161  		if x, ok := rhs.(*TypeSwitchGuard); ok && keyword == _Switch && op == Def {
  2162  			if lhs, ok := lhs.(*Name); ok {
  2163  				// switch … lhs := rhs.(type)
  2164  				x.Lhs = lhs
  2165  				s := new(ExprStmt)
  2166  				s.pos = x.Pos()
  2167  				s.X = x
  2168  				return s
  2169  			}
  2170  		}
  2171  
  2172  		return p.newAssignStmt(pos, op, lhs, rhs)
  2173  
  2174  	default:
  2175  		p.syntaxError("expected := or = or comma")
  2176  		p.advance(_Semi, _Rbrace)
  2177  		// make the best of what we have
  2178  		if x, ok := lhs.(*ListExpr); ok {
  2179  			lhs = x.ElemList[0]
  2180  		}
  2181  		s := new(ExprStmt)
  2182  		s.pos = lhs.Pos()
  2183  		s.X = lhs
  2184  		return s
  2185  	}
  2186  }
  2187  
  2188  func (p *parser) newRangeClause(lhs Expr, def bool) *RangeClause {
  2189  	r := new(RangeClause)
  2190  	r.pos = p.pos()
  2191  	p.next() // consume _Range
  2192  	r.Lhs = lhs
  2193  	r.Def = def
  2194  	r.X = p.expr()
  2195  	return r
  2196  }
  2197  
  2198  func (p *parser) newAssignStmt(pos Pos, op Operator, lhs, rhs Expr) *AssignStmt {
  2199  	a := new(AssignStmt)
  2200  	a.pos = pos
  2201  	a.Op = op
  2202  	a.Lhs = lhs
  2203  	a.Rhs = rhs
  2204  	return a
  2205  }
  2206  
  2207  func (p *parser) labeledStmtOrNil(label *Name) Stmt {
  2208  	if trace {
  2209  		defer p.trace("labeledStmt")()
  2210  	}
  2211  
  2212  	s := new(LabeledStmt)
  2213  	s.pos = p.pos()
  2214  	s.Label = label
  2215  
  2216  	p.want(_Colon)
  2217  
  2218  	if p.tok == _Rbrace {
  2219  		// We expect a statement (incl. an empty statement), which must be
  2220  		// terminated by a semicolon. Because semicolons may be omitted before
  2221  		// an _Rbrace, seeing an _Rbrace implies an empty statement.
  2222  		e := new(EmptyStmt)
  2223  		e.pos = p.pos()
  2224  		s.Stmt = e
  2225  		return s
  2226  	}
  2227  
  2228  	s.Stmt = p.stmtOrNil()
  2229  	if s.Stmt != nil {
  2230  		return s
  2231  	}
  2232  
  2233  	// report error at line of ':' token
  2234  	p.syntaxErrorAt(s.pos, "missing statement after label")
  2235  	// we are already at the end of the labeled statement - no need to advance
  2236  	return nil // avoids follow-on errors (see e.g., fixedbugs/bug274.go)
  2237  }
  2238  
  2239  // context must be a non-empty string unless we know that p.tok == _Lbrace.
  2240  func (p *parser) blockStmt(context string) *BlockStmt {
  2241  	if trace {
  2242  		defer p.trace("blockStmt")()
  2243  	}
  2244  
  2245  	s := new(BlockStmt)
  2246  	s.pos = p.pos()
  2247  
  2248  	// people coming from C may forget that braces are mandatory in Go
  2249  	if !p.got(_Lbrace) {
  2250  		p.syntaxError("expected { after " + context)
  2251  		p.advance(_Name, _Rbrace)
  2252  		s.Rbrace = p.pos() // in case we found "}"
  2253  		if p.got(_Rbrace) {
  2254  			return s
  2255  		}
  2256  	}
  2257  
  2258  	s.List = p.stmtList()
  2259  	s.Rbrace = p.pos()
  2260  	p.want(_Rbrace)
  2261  
  2262  	return s
  2263  }
  2264  
  2265  func (p *parser) declStmt(f func(*Group) Decl) *DeclStmt {
  2266  	if trace {
  2267  		defer p.trace("declStmt")()
  2268  	}
  2269  
  2270  	s := new(DeclStmt)
  2271  	s.pos = p.pos()
  2272  
  2273  	p.next() // _Const, _Type, or _Var
  2274  	s.DeclList = p.appendGroup(nil, f)
  2275  
  2276  	return s
  2277  }
  2278  
  2279  func (p *parser) forStmt() Stmt {
  2280  	if trace {
  2281  		defer p.trace("forStmt")()
  2282  	}
  2283  
  2284  	s := new(ForStmt)
  2285  	s.pos = p.pos()
  2286  
  2287  	s.Init, s.Cond, s.Post = p.header(_For)
  2288  	s.Body = p.blockStmt("for clause")
  2289  
  2290  	return s
  2291  }
  2292  
  2293  func (p *parser) header(keyword token) (init SimpleStmt, cond Expr, post SimpleStmt) {
  2294  	p.want(keyword)
  2295  
  2296  	if p.tok == _Lbrace {
  2297  		if keyword == _If {
  2298  			p.syntaxError("missing condition in if statement")
  2299  			cond = p.badExpr()
  2300  		}
  2301  		return
  2302  	}
  2303  	// p.tok != _Lbrace
  2304  
  2305  	outer := p.xnest
  2306  	p.xnest = -1
  2307  
  2308  	if p.tok != _Semi {
  2309  		// accept potential varDecl but complain
  2310  		if p.got(_Var) {
  2311  			p.syntaxError(fmt.Sprintf("var declaration not allowed in %s initializer", tokstring(keyword)))
  2312  		}
  2313  		init = p.simpleStmt(nil, keyword)
  2314  		// If we have a range clause, we are done (can only happen for keyword == _For).
  2315  		if _, ok := init.(*RangeClause); ok {
  2316  			p.xnest = outer
  2317  			return
  2318  		}
  2319  	}
  2320  
  2321  	var condStmt SimpleStmt
  2322  	var semi struct {
  2323  		pos Pos
  2324  		lit string // valid if pos.IsKnown()
  2325  	}
  2326  	if p.tok != _Lbrace {
  2327  		if p.tok == _Semi {
  2328  			semi.pos = p.pos()
  2329  			semi.lit = p.lit
  2330  			p.next()
  2331  		} else {
  2332  			// asking for a '{' rather than a ';' here leads to a better error message
  2333  			p.want(_Lbrace)
  2334  			if p.tok != _Lbrace {
  2335  				p.advance(_Lbrace, _Rbrace) // for better synchronization (e.g., go.dev/issue/22581)
  2336  			}
  2337  		}
  2338  		if keyword == _For {
  2339  			if p.tok != _Semi {
  2340  				if p.tok == _Lbrace {
  2341  					p.syntaxError("expected for loop condition")
  2342  					goto done
  2343  				}
  2344  				condStmt = p.simpleStmt(nil, 0 /* range not permitted */)
  2345  			}
  2346  			p.want(_Semi)
  2347  			if p.tok != _Lbrace {
  2348  				post = p.simpleStmt(nil, 0 /* range not permitted */)
  2349  				if a, _ := post.(*AssignStmt); a != nil && a.Op == Def {
  2350  					p.syntaxErrorAt(a.Pos(), "cannot declare in post statement of for loop")
  2351  				}
  2352  			}
  2353  		} else if p.tok != _Lbrace {
  2354  			condStmt = p.simpleStmt(nil, keyword)
  2355  		}
  2356  	} else {
  2357  		condStmt = init
  2358  		init = nil
  2359  	}
  2360  
  2361  done:
  2362  	// unpack condStmt
  2363  	switch s := condStmt.(type) {
  2364  	case nil:
  2365  		if keyword == _If && semi.pos.IsKnown() {
  2366  			if semi.lit != "semicolon" {
  2367  				p.syntaxErrorAt(semi.pos, fmt.Sprintf("unexpected %s, expected { after if clause", semi.lit))
  2368  			} else {
  2369  				p.syntaxErrorAt(semi.pos, "missing condition in if statement")
  2370  			}
  2371  			b := new(BadExpr)
  2372  			b.pos = semi.pos
  2373  			cond = b
  2374  		}
  2375  	case *ExprStmt:
  2376  		cond = s.X
  2377  	default:
  2378  		// A common syntax error is to write '=' instead of '==',
  2379  		// which turns an expression into an assignment. Provide
  2380  		// a more explicit error message in that case to prevent
  2381  		// further confusion.
  2382  		var str string
  2383  		if as, ok := s.(*AssignStmt); ok && as.Op == 0 {
  2384  			// Emphasize complex Lhs and Rhs of assignment with parentheses to highlight '='.
  2385  			str = "assignment " + emphasize(as.Lhs) + " = " + emphasize(as.Rhs)
  2386  		} else {
  2387  			str = String(s)
  2388  		}
  2389  		p.syntaxErrorAt(s.Pos(), fmt.Sprintf("cannot use %s as value", str))
  2390  	}
  2391  
  2392  	p.xnest = outer
  2393  	return
  2394  }
  2395  
  2396  // emphasize returns a string representation of x, with (top-level)
  2397  // binary expressions emphasized by enclosing them in parentheses.
  2398  func emphasize(x Expr) string {
  2399  	s := String(x)
  2400  	if op, _ := x.(*Operation); op != nil && op.Y != nil {
  2401  		// binary expression
  2402  		return "(" + s + ")"
  2403  	}
  2404  	return s
  2405  }
  2406  
  2407  func (p *parser) ifStmt() *IfStmt {
  2408  	if trace {
  2409  		defer p.trace("ifStmt")()
  2410  	}
  2411  
  2412  	s := new(IfStmt)
  2413  	s.pos = p.pos()
  2414  
  2415  	s.Init, s.Cond, _ = p.header(_If)
  2416  	s.Then = p.blockStmt("if clause")
  2417  
  2418  	if p.got(_Else) {
  2419  		switch p.tok {
  2420  		case _If:
  2421  			s.Else = p.ifStmt()
  2422  		case _Lbrace:
  2423  			s.Else = p.blockStmt("")
  2424  		default:
  2425  			p.syntaxError("else must be followed by if or statement block")
  2426  			p.advance(_Name, _Rbrace)
  2427  		}
  2428  	}
  2429  
  2430  	return s
  2431  }
  2432  
  2433  func (p *parser) switchStmt() *SwitchStmt {
  2434  	if trace {
  2435  		defer p.trace("switchStmt")()
  2436  	}
  2437  
  2438  	s := new(SwitchStmt)
  2439  	s.pos = p.pos()
  2440  
  2441  	s.Init, s.Tag, _ = p.header(_Switch)
  2442  
  2443  	if !p.got(_Lbrace) {
  2444  		p.syntaxError("missing { after switch clause")
  2445  		p.advance(_Case, _Default, _Rbrace)
  2446  	}
  2447  	for p.tok != _EOF && p.tok != _Rbrace {
  2448  		s.Body = append(s.Body, p.caseClause())
  2449  	}
  2450  	s.Rbrace = p.pos()
  2451  	p.want(_Rbrace)
  2452  
  2453  	return s
  2454  }
  2455  
  2456  func (p *parser) selectStmt() *SelectStmt {
  2457  	if trace {
  2458  		defer p.trace("selectStmt")()
  2459  	}
  2460  
  2461  	s := new(SelectStmt)
  2462  	s.pos = p.pos()
  2463  
  2464  	p.want(_Select)
  2465  	if !p.got(_Lbrace) {
  2466  		p.syntaxError("missing { after select clause")
  2467  		p.advance(_Case, _Default, _Rbrace)
  2468  	}
  2469  	for p.tok != _EOF && p.tok != _Rbrace {
  2470  		s.Body = append(s.Body, p.commClause())
  2471  	}
  2472  	s.Rbrace = p.pos()
  2473  	p.want(_Rbrace)
  2474  
  2475  	return s
  2476  }
  2477  
  2478  func (p *parser) caseClause() *CaseClause {
  2479  	if trace {
  2480  		defer p.trace("caseClause")()
  2481  	}
  2482  
  2483  	c := new(CaseClause)
  2484  	c.pos = p.pos()
  2485  
  2486  	switch p.tok {
  2487  	case _Case:
  2488  		p.next()
  2489  		c.Cases = p.exprList()
  2490  
  2491  	case _Default:
  2492  		p.next()
  2493  
  2494  	default:
  2495  		p.syntaxError("expected case or default or }")
  2496  		p.advance(_Colon, _Case, _Default, _Rbrace)
  2497  	}
  2498  
  2499  	c.Colon = p.pos()
  2500  	p.want(_Colon)
  2501  	c.Body = p.stmtList()
  2502  
  2503  	return c
  2504  }
  2505  
  2506  func (p *parser) commClause() *CommClause {
  2507  	if trace {
  2508  		defer p.trace("commClause")()
  2509  	}
  2510  
  2511  	c := new(CommClause)
  2512  	c.pos = p.pos()
  2513  
  2514  	switch p.tok {
  2515  	case _Case:
  2516  		p.next()
  2517  		c.Comm = p.simpleStmt(nil, 0)
  2518  
  2519  		// The syntax restricts the possible simple statements here to:
  2520  		//
  2521  		//     lhs <- x (send statement)
  2522  		//     <-x
  2523  		//     lhs = <-x
  2524  		//     lhs := <-x
  2525  		//
  2526  		// All these (and more) are recognized by simpleStmt and invalid
  2527  		// syntax trees are flagged later, during type checking.
  2528  
  2529  	case _Default:
  2530  		p.next()
  2531  
  2532  	default:
  2533  		p.syntaxError("expected case or default or }")
  2534  		p.advance(_Colon, _Case, _Default, _Rbrace)
  2535  	}
  2536  
  2537  	c.Colon = p.pos()
  2538  	p.want(_Colon)
  2539  	c.Body = p.stmtList()
  2540  
  2541  	return c
  2542  }
  2543  
  2544  // stmtOrNil parses a statement if one is present, or else returns nil.
  2545  //
  2546  //	Statement =
  2547  //		Declaration | LabeledStmt | SimpleStmt |
  2548  //		GoStmt | ReturnStmt | BreakStmt | ContinueStmt | GotoStmt |
  2549  //		FallthroughStmt | Block | IfStmt | SwitchStmt | SelectStmt | ForStmt |
  2550  //		DeferStmt .
  2551  func (p *parser) stmtOrNil() Stmt {
  2552  	if trace {
  2553  		defer p.trace("stmt " + p.tok.String())()
  2554  	}
  2555  
  2556  	// Most statements (assignments) start with an identifier;
  2557  	// look for it first before doing anything more expensive.
  2558  	if p.tok == _Name {
  2559  		p.clearPragma()
  2560  		lhs := p.exprList()
  2561  		if label, ok := lhs.(*Name); ok && p.tok == _Colon {
  2562  			return p.labeledStmtOrNil(label)
  2563  		}
  2564  		return p.simpleStmt(lhs, 0)
  2565  	}
  2566  
  2567  	switch p.tok {
  2568  	case _Var:
  2569  		return p.declStmt(p.varDecl)
  2570  
  2571  	case _Const:
  2572  		return p.declStmt(p.constDecl)
  2573  
  2574  	case _Type:
  2575  		return p.declStmt(p.typeDecl)
  2576  	}
  2577  
  2578  	p.clearPragma()
  2579  
  2580  	switch p.tok {
  2581  	case _Lbrace:
  2582  		return p.blockStmt("")
  2583  
  2584  	case _Operator, _Star:
  2585  		switch p.op {
  2586  		case Add, Sub, Mul, And, Xor, Not:
  2587  			return p.simpleStmt(nil, 0) // unary operators
  2588  		}
  2589  
  2590  	case _Literal, _Func, _Lparen, // operands
  2591  		_Lbrack, _Struct, _Map, _Chan, _Interface, // composite types
  2592  		_Arrow: // receive operator
  2593  		return p.simpleStmt(nil, 0)
  2594  
  2595  	case _For:
  2596  		return p.forStmt()
  2597  
  2598  	case _Switch:
  2599  		return p.switchStmt()
  2600  
  2601  	case _Select:
  2602  		return p.selectStmt()
  2603  
  2604  	case _If:
  2605  		return p.ifStmt()
  2606  
  2607  	case _Fallthrough:
  2608  		s := new(BranchStmt)
  2609  		s.pos = p.pos()
  2610  		p.next()
  2611  		s.Tok = _Fallthrough
  2612  		return s
  2613  
  2614  	case _Break, _Continue:
  2615  		s := new(BranchStmt)
  2616  		s.pos = p.pos()
  2617  		s.Tok = p.tok
  2618  		p.next()
  2619  		if p.tok == _Name {
  2620  			s.Label = p.name()
  2621  		}
  2622  		return s
  2623  
  2624  	case _Go, _Defer:
  2625  		return p.callStmt()
  2626  
  2627  	case _Goto:
  2628  		s := new(BranchStmt)
  2629  		s.pos = p.pos()
  2630  		s.Tok = _Goto
  2631  		p.next()
  2632  		s.Label = p.name()
  2633  		return s
  2634  
  2635  	case _Return:
  2636  		s := new(ReturnStmt)
  2637  		s.pos = p.pos()
  2638  		p.next()
  2639  		if p.tok != _Semi && p.tok != _Rbrace {
  2640  			s.Results = p.exprList()
  2641  		}
  2642  		return s
  2643  
  2644  	case _Semi:
  2645  		s := new(EmptyStmt)
  2646  		s.pos = p.pos()
  2647  		return s
  2648  	}
  2649  
  2650  	return nil
  2651  }
  2652  
  2653  // StatementList = { Statement ";" } .
  2654  func (p *parser) stmtList() (l []Stmt) {
  2655  	if trace {
  2656  		defer p.trace("stmtList")()
  2657  	}
  2658  
  2659  	for p.tok != _EOF && p.tok != _Rbrace && p.tok != _Case && p.tok != _Default {
  2660  		s := p.stmtOrNil()
  2661  		p.clearPragma()
  2662  		if s == nil {
  2663  			break
  2664  		}
  2665  		l = append(l, s)
  2666  		// ";" is optional before "}"
  2667  		if !p.got(_Semi) && p.tok != _Rbrace {
  2668  			p.syntaxError("at end of statement")
  2669  			p.advance(_Semi, _Rbrace, _Case, _Default)
  2670  			p.got(_Semi) // avoid spurious empty statement
  2671  		}
  2672  	}
  2673  	return
  2674  }
  2675  
  2676  // argList parses a possibly empty, comma-separated list of arguments,
  2677  // optionally followed by a comma (if not empty), and closed by ")".
  2678  // The last argument may be followed by "...".
  2679  //
  2680  // argList = [ arg { "," arg } [ "..." ] [ "," ] ] ")" .
  2681  func (p *parser) argList() (list []Expr, hasDots bool) {
  2682  	if trace {
  2683  		defer p.trace("argList")()
  2684  	}
  2685  
  2686  	p.xnest++
  2687  	p.list("argument list", _Comma, _Rparen, func() bool {
  2688  		list = append(list, p.expr())
  2689  		hasDots = p.got(_DotDotDot)
  2690  		return hasDots
  2691  	})
  2692  	p.xnest--
  2693  
  2694  	return
  2695  }
  2696  
  2697  // ----------------------------------------------------------------------------
  2698  // Common productions
  2699  
  2700  func (p *parser) name() *Name {
  2701  	// no tracing to avoid overly verbose output
  2702  
  2703  	if p.tok == _Name {
  2704  		n := NewName(p.pos(), p.lit)
  2705  		p.next()
  2706  		return n
  2707  	}
  2708  
  2709  	n := NewName(p.pos(), "_")
  2710  	p.syntaxError("expected name")
  2711  	p.advance()
  2712  	return n
  2713  }
  2714  
  2715  // IdentifierList = identifier { "," identifier } .
  2716  // The first name must be provided.
  2717  func (p *parser) nameList(first *Name) []*Name {
  2718  	if trace {
  2719  		defer p.trace("nameList")()
  2720  	}
  2721  
  2722  	if debug && first == nil {
  2723  		panic("first name not provided")
  2724  	}
  2725  
  2726  	l := []*Name{first}
  2727  	for p.got(_Comma) {
  2728  		l = append(l, p.name())
  2729  	}
  2730  
  2731  	return l
  2732  }
  2733  
  2734  // The first name may be provided, or nil.
  2735  func (p *parser) qualifiedName(name *Name) Expr {
  2736  	if trace {
  2737  		defer p.trace("qualifiedName")()
  2738  	}
  2739  
  2740  	var x Expr
  2741  	switch {
  2742  	case name != nil:
  2743  		x = name
  2744  	case p.tok == _Name:
  2745  		x = p.name()
  2746  	default:
  2747  		x = NewName(p.pos(), "_")
  2748  		p.syntaxError("expected name")
  2749  		p.advance(_Dot, _Semi, _Rbrace)
  2750  	}
  2751  
  2752  	if p.tok == _Dot {
  2753  		s := new(SelectorExpr)
  2754  		s.pos = p.pos()
  2755  		p.next()
  2756  		s.X = x
  2757  		s.Sel = p.name()
  2758  		x = s
  2759  	}
  2760  
  2761  	if p.tok == _Lbrack {
  2762  		x = p.typeInstance(x)
  2763  	}
  2764  
  2765  	return x
  2766  }
  2767  
  2768  // ExpressionList = Expression { "," Expression } .
  2769  func (p *parser) exprList() Expr {
  2770  	if trace {
  2771  		defer p.trace("exprList")()
  2772  	}
  2773  
  2774  	x := p.expr()
  2775  	if p.got(_Comma) {
  2776  		list := []Expr{x, p.expr()}
  2777  		for p.got(_Comma) {
  2778  			list = append(list, p.expr())
  2779  		}
  2780  		t := new(ListExpr)
  2781  		t.pos = x.Pos()
  2782  		t.ElemList = list
  2783  		x = t
  2784  	}
  2785  	return x
  2786  }
  2787  
  2788  // typeList parses a non-empty, comma-separated list of types,
  2789  // optionally followed by a comma. If strict is set to false,
  2790  // the first element may also be a (non-type) expression.
  2791  // If there is more than one argument, the result is a *ListExpr.
  2792  // The comma result indicates whether there was a (separating or
  2793  // trailing) comma.
  2794  //
  2795  // typeList = arg { "," arg } [ "," ] .
  2796  func (p *parser) typeList(strict bool) (x Expr, comma bool) {
  2797  	if trace {
  2798  		defer p.trace("typeList")()
  2799  	}
  2800  
  2801  	p.xnest++
  2802  	if strict {
  2803  		x = p.type_()
  2804  	} else {
  2805  		x = p.expr()
  2806  	}
  2807  	if p.got(_Comma) {
  2808  		comma = true
  2809  		if t := p.typeOrNil(); t != nil {
  2810  			list := []Expr{x, t}
  2811  			for p.got(_Comma) {
  2812  				if t = p.typeOrNil(); t == nil {
  2813  					break
  2814  				}
  2815  				list = append(list, t)
  2816  			}
  2817  			l := new(ListExpr)
  2818  			l.pos = x.Pos() // == list[0].Pos()
  2819  			l.ElemList = list
  2820  			x = l
  2821  		}
  2822  	}
  2823  	p.xnest--
  2824  	return
  2825  }
  2826  
  2827  // Unparen returns e with any enclosing parentheses stripped.
  2828  func Unparen(x Expr) Expr {
  2829  	for {
  2830  		p, ok := x.(*ParenExpr)
  2831  		if !ok {
  2832  			break
  2833  		}
  2834  		x = p.X
  2835  	}
  2836  	return x
  2837  }
  2838  
  2839  // UnpackListExpr unpacks a *ListExpr into a []Expr.
  2840  func UnpackListExpr(x Expr) []Expr {
  2841  	switch x := x.(type) {
  2842  	case nil:
  2843  		return nil
  2844  	case *ListExpr:
  2845  		return x.ElemList
  2846  	default:
  2847  		return []Expr{x}
  2848  	}
  2849  }
  2850  

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